1. Field of the Invention
The present invention relates to a pressure swing adsorption-type oxygen concentration apparatus using an adsorbent that preferentially adsorbs nitrogen rather than oxygen and, in particular, it relates to a medical oxygen-concentration apparatus used in oxygen-inhalation therapy that is a treatment for respiratory diseases.
2. Description of the Related Art
In recent years, the number of patients that suffer from respiratory diseases such as asthma, pulmonary emphysema, chronic bronchitis and the like has been growing. One of the most effective therapy methods for such respiratory diseases is oxygen inhalation therapy that allows the patient to inhale an oxygen-concentrated gas or oxygen-enriched air. As a source of the oxygen-concentrated gas or the oxygen-enriched air supplied to the patient (referred to as the “oxygen supply source” in this specification), an oxygen concentration apparatus, liquid oxygen, an oxygen concentrated gas tank and the like are well-known but, among others, in view of convenience in use and ease of maintenance and management, the oxygen concentration apparatus is mainly used in home oxygen therapy.
Though a membrane type oxygen concentration apparatus that uses a membrane for selectively permeating oxygen is known, a pressure swing adsorption type oxygen concentration apparatus that uses an adsorbent that preferentially adsorbs nitrogen, and that can provide higher concentration of oxygen, is more widely used.
Known methods for supplying the oxygen concentrated gas generated in the oxygen concentration apparatus include: a method for supplying the oxygen concentrated gas at a constant flow rate continuously; and a method for supplying the oxygen concentrated gas only in an inspiratory phase, or in a portion of the inspiratory phase, in synchronization with the patient's respiration.
When the oxygen concentrated gas at a constant flow rate is supplied continuously, the oxygen concentration apparatus is provided with a flow rate adjuster for supplying the oxygen concentrated gas at a prescribed constant flow rate to the patient. This flow rate adjuster may be an orifice type flow rate adjuster, a flow rate adjuster using a needle valve, and a feedback type flow rate adjuster using a flow rate sensor. The orifice type flow rate adjuster has a plurality of orifices of different diameters so that one of the plurality of orifices can be selected to obtain a desired flow rate under the pressure condition at the upstream of the orifice. The feedback type flow rate adjuster controls a degree of opening of a throttle valve based on a measurement value by the flow rate sensor.
Further, Japanese Unexamined Patent Publication No. S61-131756 and Japanese Examined Patent Publication No. H03-22185 disclose an oxygen supply method for supplying oxygen concentrated gas only in an inspiratory phase, or in a portion of the inspiratory phase, in synchronization with the patient's respiration, and a pressure swing adsorption type oxygen concentration apparatus with this respiratory synchronous oxygen supplying method.
Still further, Japanese Unexamined Patent Publication No. 2001-187145, Japanese Unexamined Patent Publication No. 2003-144549, and Japanese Unexamined Patent Publication No. 2003-144550 disclose a mechanical pressure regulating valve having a piston and a spring used in the oxygen supply method in a continuous or respiratory synchronous intermittent manner described above.
Still further, Japanese Unexamined Patent Publication No. 2000-352482, Japanese Unexamined Patent Publication No. 2002-121010, Japanese Unexamined Patent Publication No. H07-136272, and Japanese Unexamined Patent Publication No. 2002-45424 disclose a battery-driven mobile or portable oxygen concentration apparatus that extends the area of activity of the patient and contributes to an improved quality of life (QOL).
On the other hand, when the oxygen concentration apparatus supplies the oxygen concentrated gas to the patient, the oxygen concentration of the oxygen concentrated gas may reduce due to degradation of the adsorbent, failure of the concentration apparatus itself and so on. The patient cannot obtain a sufficient therapeutic effect with reduced oxygen concentration of the oxygen concentrated gas and, therefore, it is desirable to provide the oxygen concentration apparatus with an oxygen concentration sensor for measuring the concentration of the oxygen concentrated gas.
While a zirconia type oxygen concentration sensor has been typically used as the oxygen concentration sensor for measuring the oxygen concentration of the oxygen concentrated gas, Japanese Unexamined Patent Publication No. 2002-214012 and Japanese Unexamined Patent Publication No. 2003-135601 disclose an ultrasonic type gas concentration and flow rate measuring apparatus.
Hereinafter, a principle of gas concentration measurement by the ultrasonic type gas concentration and flow rate measuring means will be described.
Two ultrasonic transducers that can transmit and receive ultrasonic waves with each other are disposed in an opposed manner in a line through which a product gas flows so that the ultrasonic waves can be transmitted and received in the forward direction of the gas flow. Assuming that a sound velocity observed in this case is V1, a sound velocity in immobile gas is C and a flow velocity of the gas in the line is V, V1 can be expressed by the following formula (1):V1=C+V  (1)
Then, a sound velocity V2 observed when the ultrasonic waves are transmitted and received in the reverse direction of the gas flow can be expressed by the following formula (2):V2=C−V  (2)
Therefore, even if the flow velocity V of the gas is unknown, the flow velocity V of the gas can be canceled by adding the formulas (1) and (2) and, as a result, only the sound velocity C in the immobile gas can be calculated by the following formula (3):C=(V1+V2)/2  (3)
Further, assuming that a gas temperature is T, a ratio of specific heat of the gas is k, a gas constant is R and an average molecular weight of the gas is M, it is known that the sound velocity C in the immobile gas can be expressed by the following formula (4):
                    C        =                              kRT            M                                              (        4        )            
In the formula (4), k and R are constant and the value of C can be obtained by the formula (3) and, therefore, if only the gas temperature T is measured, the formula (4) can be transformed into the formula (5) to obtain the average molecular weight M of the gas:M=kRT/C2  (5)
Thus, for example, if the measured gas is a two-component gas consisting of oxygen and nitrogen, assuming that the oxygen concentration is x, the nitrogen concentration is 1−x, a molecular weight of the oxygen is 32 and a molecular weight of the nitrogen is 28, the oxygen concentration x can be determined by using the relationship of the following formula (6):32x+28(1−x)=M  (6)
Further, a principle of flow rate measurement in the ultrasonic type gas concentration flow rate measuring apparatus in which two ultrasonic transducers are arranged in an opposed manner is as follows.
By using the formulas (1) and (2) described above, even if the sound velocity C in the immobile gas is unknown, the flow velocity V of the gas can be obtained by the following formula (7):V=(V1−V2)/2  (7)
Then, if the flow velocity V of the gas can be obtained, the flow rate of the gas can be obtained easily by multiplying it by a cross sectional area of the line through which the gas flows.